Various kinds of motors, such as a servo motor used with a machine tool etc., are driven by a motor control system including a PWM controlled inverter circuit. If the machine tool uses a cutting fluid, the motor may get wet with the cutting fluid and the cutting water may enter the motor. The electrical insulation of the motor may accordingly be deteriorated. The electrical insulation deterioration of the motor is progressing gradually and finally results in a ground fault. The ground fault of motor may cause an earth leakage circuit to trip and damage the motor control system, which may result in a failure of a system using the motor. The system failure exerts great influence on the production line of a factory. Accordingly, from the viewpoint of preventive maintenance, a system, which is capable of detecting the electrical insulation deterioration of motors, is needed.
Examples of conventional methods of detecting the electrical insulation deterioration of motor are as follows:
(1) To use an electrical insulation resistance meter;
(2) To use a leakage current detector;
(3) To insert a series circuit formed by a capacitor and a resistance between a ground and one of a pair of direct current (hereinafter referred to as DC) input portions or between positive and negative input portions of a PWM controlled inverter circuit, and dispose a detection circuit that detects the potential difference between both ends of the resistance in the series circuit, so that the electrical insulation deterioration of a motor can be detected without stopping the motor, as disclosed in Japanese Patent Application Publication No. 2005-201669 or JP2005-201669A;
(4) To ground a DC input portion of a PWM controlled inverter circuit, apply a direct current voltage across an inverter system to a motor through a semiconductor device included in the inverter circuit, and detect an electrical insulation resistance of the motor by measuring the direct current voltage and the direct current of the inverter system, as disclosed in Japanese Patent Application Publication No. 1994-94762 or JP1994-94762A; and
(5) To determine electrical insulation deterioration by using a charged voltage across a smoothing capacitor when all of a plurality of semiconductor switch devices constituting an inverter circuit are in a non-conductive state, as disclosed in Japanese Patent Application Publication No. 2005-16958 and Japanese Patent Application Publication No. 2005-110400 [JP2005-16958A and JP1005-110400A].
In these publicly known methods, the charged voltage across the smoothing capacitor is applied to a motor through a switch provided separately from the semiconductor switches constituting the inverter circuit.
In the above-mentioned method (1) which uses the electrical insulation resistance meter, the deterioration of electrical insulation is detected based on the value of electrical insulation resistance measured by connecting the electrical insulation resistance meter between a winding of the motor and a ground to measure the value of electrical insulation resistance with electric wiring between a motor and a motor control system being removed. However, this method requires many man-hours for removing wiring of the motor.
In the above-mentioned method (2) which uses the leakage current detector, the electrical insulation deterioration of a motor on a detected value obtained by the leakage current detector disposed between the power supply inputs of a motor control system. However, in the motor control system including the PWM controlled inverter circuit, since leakage current flows through the power supply due to a switching operation of semiconductor devices included in the inverter circuit, it is difficult to properly detect the electrical insulation deterioration of the motor.
In the above-mentioned method (3), since current flowing through the detection circuit is affected by a leakage current flowing through a power supply or flowing through stray capacitances of a motor and a wiring cable, a detection error is likely to occur in detecting the electrical insulation deterioration.
In the above-mentioned method (4), a direct-current input portion of the inverter circuit is grounded and the direct current voltage applied to the inverter device is applied to the motor through the semiconductor devices included in the inverter system. Then, the electrical insulation resistance of the motor is detected by measuring the DC input voltage and DC input current of the inverter circuit at that time. In this method, however, if the electrical insulation of the motor is deteriorated and the ground fault has occurred, the direct current voltage of the inverter circuit may be short-circuited when the semiconductor devices included in the inverter circuit are in a conductive state. As a result, the semiconductor devices may be damaged. A small current flows into a detector for measuring a direct current when detecting the electrical insulation deterioration while a large current flows at the time of usual motor control. Accordingly, the current detector is required not only to tolerate high current but also to be capable of detecting very small current with sufficient accuracy. However, an ordinary current detector cannot obtain such high detecting accuracy, and it is difficult to increase the detecting accuracy of an ordinary current detector.
In the above-mentioned method (5) of the electrical insulation deterioration determination as disclosed in JP2005-16958A and JP2005-110400A, since the charged voltage across the smoothing capacitor is applied to the motor through a switch provided separately from the semiconductor switches constituting the inverter circuit, the number of components is inevitably increased. Further, it is difficult o know the cause of the electrical insulation deterioration of a motor by the conventional methods.